|erm.. no.... you get one service to one router. You can't run the single feed to two (or more) routers and all of them active/working at the same time.|
However as long as the single feed is clearly stable and available at both locations... you probably could use a router at either lcation - but only one location at a time.
But... generally dsl to router connection is best if it's very short, ideally directly from the master socket. Whilst can be distributed around a home (with that wiring having the phone outlets in a series arrangement rather than the more common star arrangement; and the latter is very common in many homes), even that can be less than ideal. And you want to send it over a 200ft cable, of unknown quality and state - hard to say if it will work well enough - if at all...
You could try it by connecting your current router to the barn phone line, and connecting the dsl service to that phone line? If it works, then possibly you can try with your current router connected as now, and a second router in the barn. But again you will only be able to use one router at a time; the other must be powered down, or at least disconnected from the dsl service.
Where you may hit a snag.. is the router supplied by AT&T. Possibly they have locked their service down to their router. By that I mean you don't know your actual account login/password and thus you may not be able to login to your dsl service from another router - especially if it's one you obtain from other than AT&T.
message edited by trvlr